You made that argument in the Space thread, and that's one of the things I wished I'd had time to address...now I'll make time.

I don't know if a chimpanzee has ever signed "I don't want to die". You may be right that they don't connect the dots in the sense of predicting their own demise, though the empathic abilities of social sentients suggests that, having seen death come to other members of the tribe, they may be able to recognize death as something that could happen to them too.

But I'm not sure if awareness of mortality should be such a key criteria of sentience. What of a hypothetical communal organism that's effectively immortal? I don't see how I can make the statement that individual death is a requirement for sentience to develop--where's the logical connection?

We tend to place great emphasis on language skills and communication as a feature of sentience, and sometimes overlook that other forms of communication exist. The essence of being a social primate is constant interaction with fellow primates through continuous signaling of intentions and emotional state through action and expression and other nonverbal signs. Primates never stop communicating whenever two or more gather.

I'm not sure how that affects the observation that the primate cousins can't communicate across the generations. A tribe provides continuity across generations, and it's possible that, to the extent that the tribe forms a sort of group mind, there is communication through time. The tribe's customs and conventions and knowledge of the surrounding environment persists even as individual are added and subtracted. Without language we won't see granny chimp imparting any wisdom to the young whippersnapper chimps, but I suspect that the mechanism is there in a subtler form.

It's interesting that studies of human child development show that parents "bootstrap" a human mind out of an infant brain primarily through emotional interactions that begin at birth. The cousins keep up with us through the early phases of that process, but drop out when it reaches the point of requiring language skills. Parents teach infants lessons such as cause-and-effect with primarily emotional games such as back-and-forth smiling--the infant learns "when I smile Mommy smiles...I can make Mommy smile!" and progresses from causality into intention. Similarly, parents teach object persistence with simple sleight-of-hand games, and build on that to teach that other humans are also persistent beings with the universal game of "peekaboo"; all of which have a high emotional content that makes the lesson sink in.

In other words, human parents and infants communicate at first in an entirely emotional language, so I'm inclined to credit the other primates with at least that much communication ability. The prerequisites for language are all there in that first-phase communication, and like you, I find the thought of someday gifting chimps and gorillas with speech to be thoroughly fascinating.

'Course, the first words we hear are liable to begin "you sonsabitches!" followed by a long catalog of our bestial behavior toward beings we ought to recognize as much like ourselves. The downside of bringing speech to other species is that then we'll get to hear what they think of us.